[PATCH] usb: gadget: function: printer: Replace strlcpy with strscpy

[PATCH] usb: gadget: function: printer: Replace strlcpy with strscpy

[Azeem Shaikh]

strlcpy() reads the entire source buffer first.
This read may exceed the destination size limit.
This is both inefficient and can lead to linear read
overflows if a source string is not NUL-terminated [1].
In an effort to remove strlcpy() completely [2], replace
strlcpy() here with strscpy().

Direct replacement is safe here since return value of -E2BIG
is used to check for truncation instead of sizeof(dest).

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
[2] https://github.com/KSPP/linux/issues/89

Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
---
 drivers/usb/gadget/function/f_printer.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/function/f_printer.c b/drivers/usb/gadget/function/f_printer.c
index 28db3e336e7d..10dc7c55c25f 100644
--- a/drivers/usb/gadget/function/f_printer.c
+++ b/drivers/usb/gadget/function/f_printer.c
@@ -1211,8 +1211,8 @@ static ssize_t f_printer_opts_pnp_string_show(struct config_item *item,
 	if (!opts->pnp_string)
 		goto unlock;
 
-	result = strlcpy(page, opts->pnp_string, PAGE_SIZE);
-	if (result >= PAGE_SIZE) {
+	result = strscpy(page, opts->pnp_string, PAGE_SIZE);
+	if (result == -E2BIG) {
 		result = PAGE_SIZE;
 	} else if (page[result - 1] != '\n' && result + 1 < PAGE_SIZE) {
 		page[result++] = '\n';
-- 
2.41.0.162.gfafddb0af9-goog

Re: [PATCH] usb: gadget: function: printer: Replace strlcpy with strscpy

[Kees Cook]

On Tue, Jun 13, 2023 at 12:43:41AM +0000, Azeem Shaikh wrote:
> strlcpy() reads the entire source buffer first.
> This read may exceed the destination size limit.
> This is both inefficient and can lead to linear read
> overflows if a source string is not NUL-terminated [1].
> In an effort to remove strlcpy() completely [2], replace
> strlcpy() here with strscpy().
> 
> Direct replacement is safe here since return value of -E2BIG
> is used to check for truncation instead of sizeof(dest).

Perhaps say "PAGE_SIZE" here instead of sizeof(dest)?

> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> [2] https://github.com/KSPP/linux/issues/89
> 
> Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
> ---
>  drivers/usb/gadget/function/f_printer.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/f_printer.c b/drivers/usb/gadget/function/f_printer.c
> index 28db3e336e7d..10dc7c55c25f 100644
> --- a/drivers/usb/gadget/function/f_printer.c
> +++ b/drivers/usb/gadget/function/f_printer.c
> @@ -1211,8 +1211,8 @@ static ssize_t f_printer_opts_pnp_string_show(struct config_item *item,
>  	if (!opts->pnp_string)
>  		goto unlock;
>  
> -	result = strlcpy(page, opts->pnp_string, PAGE_SIZE);
> -	if (result >= PAGE_SIZE) {
> +	result = strscpy(page, opts->pnp_string, PAGE_SIZE);
> +	if (result == -E2BIG) {

I think "< 1" might be a better test here.

>  		result = PAGE_SIZE;
>  	} else if (page[result - 1] != '\n' && result + 1 < PAGE_SIZE) {
>  		page[result++] = '\n';
> -- 
> 2.41.0.162.gfafddb0af9-goog
> 
> 

-Kees

-- 
Kees Cook

Re: [PATCH] usb: gadget: function: printer: Replace strlcpy with strscpy

[Azeem Shaikh]

On Tue, Jun 13, 2023 at 3:30 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Tue, Jun 13, 2023 at 12:43:41AM +0000, Azeem Shaikh wrote:
> > strlcpy() reads the entire source buffer first.
> > This read may exceed the destination size limit.
> > This is both inefficient and can lead to linear read
> > overflows if a source string is not NUL-terminated [1].
> > In an effort to remove strlcpy() completely [2], replace
> > strlcpy() here with strscpy().
> >
> > Direct replacement is safe here since return value of -E2BIG
> > is used to check for truncation instead of sizeof(dest).
>
> Perhaps say "PAGE_SIZE" here instead of sizeof(dest)?
>

Ack.

> >
> > [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> > [2] https://github.com/KSPP/linux/issues/89
> >
> > Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
> > ---
> >  drivers/usb/gadget/function/f_printer.c |    4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/usb/gadget/function/f_printer.c b/drivers/usb/gadget/function/f_printer.c
> > index 28db3e336e7d..10dc7c55c25f 100644
> > --- a/drivers/usb/gadget/function/f_printer.c
> > +++ b/drivers/usb/gadget/function/f_printer.c
> > @@ -1211,8 +1211,8 @@ static ssize_t f_printer_opts_pnp_string_show(struct config_item *item,
> >       if (!opts->pnp_string)
> >               goto unlock;
> >
> > -     result = strlcpy(page, opts->pnp_string, PAGE_SIZE);
> > -     if (result >= PAGE_SIZE) {
> > +     result = strscpy(page, opts->pnp_string, PAGE_SIZE);
> > +     if (result == -E2BIG) {
>
> I think "< 1" might be a better test here.
>

Curious, why "< 1" instead of "< 0"?

> >               result = PAGE_SIZE;
> >       } else if (page[result - 1] != '\n' && result + 1 < PAGE_SIZE) {
> >               page[result++] = '\n';
> > --
> > 2.41.0.162.gfafddb0af9-goog
> >
> >
>
> -Kees
>
> --
> Kees Cook

Re: [PATCH] usb: gadget: function: printer: Replace strlcpy with strscpy

[Kees Cook]

On Wed, Jun 14, 2023 at 10:12:14AM -0400, Azeem Shaikh wrote:
> On Tue, Jun 13, 2023 at 3:30 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Tue, Jun 13, 2023 at 12:43:41AM +0000, Azeem Shaikh wrote:
> [...]
> > > -     result = strlcpy(page, opts->pnp_string, PAGE_SIZE);
> > > -     if (result >= PAGE_SIZE) {
> > > +     result = strscpy(page, opts->pnp_string, PAGE_SIZE);
> > > +     if (result == -E2BIG) {
> >
> > I think "< 1" might be a better test here.
> 
> Curious, why "< 1" instead of "< 0"?
> 
> > >               result = PAGE_SIZE;
> > >       } else if (page[result - 1] != '\n' && result + 1 < PAGE_SIZE) {

It's for this case above where "result" may be used in an array index,
and if it's 0 or less, there will be a negative array index (due to the "-
1"). So, here, it needs to be "< 1" instead of the more traditional "< 0".

-- 
Kees Cook

Re: [PATCH] usb: gadget: function: printer: Replace strlcpy with strscpy

[Azeem Shaikh]

On Wed, Jun 14, 2023 at 1:51 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Wed, Jun 14, 2023 at 10:12:14AM -0400, Azeem Shaikh wrote:
> > On Tue, Jun 13, 2023 at 3:30 PM Kees Cook <keescook@chromium.org> wrote:
> > >
> > > On Tue, Jun 13, 2023 at 12:43:41AM +0000, Azeem Shaikh wrote:
> > [...]
> > > > -     result = strlcpy(page, opts->pnp_string, PAGE_SIZE);
> > > > -     if (result >= PAGE_SIZE) {
> > > > +     result = strscpy(page, opts->pnp_string, PAGE_SIZE);
> > > > +     if (result == -E2BIG) {
> > >
> > > I think "< 1" might be a better test here.
> >
> > Curious, why "< 1" instead of "< 0"?
> >
> > > >               result = PAGE_SIZE;
> > > >       } else if (page[result - 1] != '\n' && result + 1 < PAGE_SIZE) {
>
> It's for this case above where "result" may be used in an array index,
> and if it's 0 or less, there will be a negative array index (due to the "-
> 1"). So, here, it needs to be "< 1" instead of the more traditional "< 0".
>

Makes sense. Sent out a v2.